Free Millwright Pumps, Seals, and Packing Questions and Answers

Question 1

What is the primary cause of cavitation in a centrifugal pump?

Accepted Answer

Suction pressure dropping below the liquid's vapor pressure

Answer

Cavitation occurs when the pressure of the liquid at the pump's suction drops below its vapor pressure. This causes the liquid to flash into vapor bubbles, which then collapse violently as they move into higher-pressure areas of the pump, causing noise, vibration, and significant damage to the impeller.

Question 2

In a typical mechanical seal, what are the two primary, lapped sealing surfaces called?

Accepted Answer

The stationary and rotating faces

Answer

A mechanical seal functions by using two ultra-flat, lapped faces that run against each other. One face is fixed to the stationary pump housing, and the other rotates with the shaft, creating a seal with a microscopic film of fluid between them.

Question 3

When installing new packing rings into a stuffing box, how should the joints of consecutive rings be oriented?

Accepted Answer

Staggered by 90 or 180 degrees

Answer

The joints or skives of each packing ring should be staggered, typically by 90 or 180 degrees, relative to the previous ring. Aligning the joints would create a direct path for fluid to leak along the shaft, defeating the purpose of the packing.

Question 4

What is the term for the condition where the pump and motor shafts are parallel but their centerlines are offset from each other?

Accepted Answer

Parallel misalignment

Answer

This condition is known as parallel misalignment. The shafts are parallel to each other in both the vertical and horizontal planes, but their centerlines are not coaxial. The other primary type of misalignment is angular, where the shafts are at an angle to each other.

Question 5

What is the function of a lantern ring (or seal cage) in a pump's stuffing box?

Accepted Answer

To distribute sealing or flushing liquid to the packing

Answer

A lantern ring is a perforated ring placed between packing rings. Its purpose is to create a space for a liquid (usually from an external source or the pump discharge) to be introduced into the stuffing box to cool and lubricate the packing, or to flush abrasives away.

Blog Practice Test

Blog Practice Test

Free Millwright Pumps, Seals, and Packing Questions and Answers